1. Field of the Invention
The present invention relates generally to surface inspection apparatuses, and is directed specifically to a surface inspection apparatus that inspects whether or not there are such defects as flaws, dirty, hit marks, etc., on the surface of an object which is, for example, a bottle cap or beer can end (bottom and top, or the like).
2. Description of the Prior Art
A cap for a bottle or an end for a beer can or the like is generally made of plastic material or metal whereas its flat surface (its top or bottom surface) is round-shaped. Further, on such cap or end it is normal that a plurality of concentric circular-shaped (ring-shaped) or spiral-shaped unevenness or grooves are formed in order that the cap or the like sufficiently withstands the pressure variations as caused by the liquids within such containers, or in other words in order to increase the strength of the cap or the like.
FIG. 1A is a top plan view of a cap 1 for a bottle that is made of metal as one example of an object to be inspected, and FIG. 1B is a cross section diagram taken along the line B-B in FIG. 1A. In the figures, a, b and c are concentric circular-shaped unevennesses, whereas a is a ring-shaped protrusion that protrudes upwards from the surface of cap 1 while b and c are respectively ring-shaped edge lines of steps that are formed on the surface of cap 1 at the inside portion of the ring-shaped protrusion a.
FIG. 2 is a schematic diagram which shows an example of a surface inspection apparatus of the conventional art that inspects the existence or not of flaws such as surface scratches or the like on the surface of cap 1 for the bottle as the inspected object as shown on FIG. 1. In FIG. 2, there is provided a light source 2 such as a lamp that irradiates the surface of cap 1. In this case, since the cap 1 has on its surface unevenness, in order to irradiate the vertical surfaces or walls thereof also, the light source 2 is placed off to an upper angled location of the cap 1. There is provided a photoelectric conversion sensor 3 such as a video camera which generates a video signal upon receiving the reflection light from the surface of cap 1 as the light is irradiated thereon from the light source 2. This video camera 3 is located above the cap 1 in a manner that the optical light axis thereof coincides to the center axis X of cap 1 which is perpendicular to the surface of cap 1 and at the same time passes the center point 0 of the cap 1. An electronic processor 4 is provided which is formed of a computer, etc., and analyzes and processes the video signal from the video camera 3 and then judges the good or bad of the surface of cap 1.
Further, FIG. 3 is a plan view that shows the relation of cap 1 and the light source 2 as shown on FIG. 2.
Now then, the light from the light source 2, that is placed off to the upper-angled location of the cap 1, is irregularly reflected on the surface of cap 1, and a part of such irregularly reflected light is picked up by the photoelectric conversion sensor or the video camera 3 which then generates the video signal of the image of the surface of cap 1. This video signal is then processed by the electronic processor 4 so that such flaws as scratches or dirties that appear as abnormally dark or extremely bright spots on the surface of cap 1 are detected so as to make the judgement of whether the surface of the cap 1 is good or bad.
In this case, as shown in FIG. 3, for the light of the center portion among the lights emitted from the light source 2, that is, the light which goes on substantially along an optical axis A of the light source 2, portions a', b'and c' of the ring-shaped protrusions a as well as the ring-shaped boundaries b and c on the surface of the cap 1, which portions a', b' and c' are positioned at the side to the light source 2 with respect to the center 0 of the cap 1 and shown by thick lines, and a portion a" of the ring-shaped protrusion a which is located at the side opposite to the portion a' with respect to the center O, are portions that stand up like walls as will be clear from Figs. lB and 2. The light from the light source 2 is irregularlly reflected on these portions a', b',c' and a" drastically excessive as compared with the other portions and as a matter of fact, such irregularly reflected lights on such portions a', b', c' and a" are directly picked up by the video camera 3. On the other hand, the majority of the lights that are irregularly reflected on the portions of the same ring-shaped protrusion a as well as boundaries b, c, which stand up like a wall except the portions a', b', c' and a", propagate in the directions as shown by arrow marks d as in FIG. 3, and do not directly reach the video camera 3. It is without question that a part of the light, which is irregularly reflected under normal conditions on the flat surface portion of cap 1, is picked up by the video camera 3 which then produces the video signal of cap 1. The video signal from the video camera 3 is supplied to the electronic processor 4 which processes the video signal same as aforementioned to judge the existence or not of flaws on cap 1 or rather whether the cap 1 is good or bad. Further, the angular range ARC of the arc portions a', a", b' and c' is smaller than 90.degree. as centered against the optical axis A.
As mentioned above at the conventional apparatuses, the lights, that are irregularly reflected on the portions a', b', c' and a" on the surface of cap 1, contain drastically excessive irregularly reflection lights in comparision to those of the other portions and since such irregularly reflected lights directly reach the video camera 3, such above mentioned portions a', b', c' and a" appear to be distinctly brighter than the other portions in the video camera 3. Thus, it not only causes the inspection of the existence or not of flaws at these portions a', b', c' and a" to be impossible, but also causes bad influences on the inspection of the other portions. As attempts to resolve such problems, inspection is conducted only on other portions by excluding these portions that cause extreme irregular reflections on the surface of cap 1 or otherwise, while attempting to select various angles of irradiation from the light source 2 onto the surface of cap 1, by dividing the surface of the same cap 1 into a plurality of portions, and thereby upon repeating the inspection for a number of times, which requires more handling care and great time loss.